The essential elements of a camera are the body, the lens assembly, the shutter which allows light to enter for a sufficiently short period of time, and a photosensitive film for recording the image. In order to produce very short exposures, a lens assembly with a great aperture must be used. For a high quality camera, this means a complex lens assembly in which aberrations of the elements tend to compensate each other.
Camera lens assemblies are characterised by two parameters. One of the two is the focal distance f which depends on the one hand on the refractive indices of the external medium and the lenses which constitute it, and on the other hand, the radius of curvature of such lenses. The second parameter which characterises a photographic lens assembly is its aperture, usually expressed with respect to the focal distance. An f/8 lens, for example, has an aperture equal to an eighth of the focal distance. The intensity of the image formed on the film is a function of the lens aperture. Practically all cameras have a lens assembly fitted with a diaphragm in order to be able to modify the aperture thereof.
Two families of cameras currently exist on the market. One of these families is formed by box cameras with interchangeable lens assemblies onto which lenses with different focal distances can be fitted as required. The other family concerns compact cameras which appeared on the market around fifteen years ago. These cameras are generally characterised by their compactness and great simplicity of use. They include a lens assembly which cannot be removed and whose focal distance may be fixed or variable.
In practice, when the lens assembly has a fixed focal distance, the lenses of which it is formed are arranged in the inner volume of the compact box. This method of construction allows sealing of the camera to be assured. Conversely, the user has only one magnification available defined by the optical geometry of the lens assembly. Lens assemblies with variable focal distances, also called zoom lenses, offer the user a continuous magnification range, but project from the front of the camera, which is detrimental to the sealing of the camera.
In order to overcome this problem, integrating an optical device forming a multiple focal distance lens assembly in the box of compact cameras has been proposed, a known embodiment thereof being shown in FIG. 1 annexed hereto. Such a device designated as a whole by the general numerical reference 1, conventionally includes two pairs of parallel lenses opposite each other, 2 and 4. The refractive indices and the radii of curvature of the faces of these lenses 2, 4 are adjusted so as to obtain the desired focal distances, represented by the directions A and B in FIG. 1. In practice, one of these focal distances which can be, for example, 38 or 50 mm, must allow standard photographs to be taken, whereas the other focal distance provides a telephoto type magnification. As focal distances A and B defined by the two pairs of lenses 2, 4 are perpendicular to each other, optical device 1 need only be pivoted by a quarter revolution to pass from a standard mode photograph to a telephoto mode photograph.
Device 1 above has a double advantage. On the one hand, it can be easily integrated into the inner volume of a compact box camera, which allows the aforementioned sealing problems to be avoided. On the other hand, it offers the user the possibility of choosing between two different magnifications.
Optical device 1 also has, however, certain drawbacks. As a single lens does not generate a high quality image, lenses 2, 4 are preferably arranged in pairs in order to compensate chromatic aberrations. As it passes through device 1, the light is refracted at each lens/air interface, so that it is difficult to produce a suitably focused image. Lenses 2, 4 must also be cut and assembled with great precision, which substantially increases the cost price of such structures.